That's why I said theoretically it always had to be Eowyn, as a "theoretical being" isn't the answer to the prophecy. Tolkien intended it to be Eowyn. (Just to clarify incase I'm confusing the hell out of you, yes I agree with you )

The hobbit Merry distracted the Witch-King, and Eowyn took advantage of the situation and did what no man could do – she killed the Witch-King

I don't the general audience appreciate Merry and Eowyn's combined role. Tolkien spent long and hard thinking of these plots and after this discussion it is clear that, for us more hardcore fans anway, a better scene was required in the films.